Users who want to share a single public IP address for outgoing connections
between several hosts traditionally reach for SNAT. However, SNAT requires
state keeping on the node(s) performing the NAT.
A stateless alternative exists, where a single IP address used for egress
can be shared between several hosts by partitioning the available ephemeral
port range. In such a setup:
1. Each host gets assigned a disjoint range of ephemeral ports.
2. Applications open connections from the host-assigned port range.
3. Return traffic gets routed to the host based on both, the destination IP
and the destination port.
An application which wants to open an outgoing connection (connect) from a
given port range today can choose between two solutions:
1. Manually pick the source port by bind()'ing to it before connect()'ing
the socket.
This approach has a couple of downsides:
a) Search for a free port has to be implemented in the user-space. If
the chosen 4-tuple happens to be busy, the application needs to retry
from a different local port number.
Detecting if 4-tuple is busy can be either easy (TCP) or hard
(UDP). In TCP case, the application simply has to check if connect()
returned an error (EADDRNOTAVAIL). That is assuming that the local
port sharing was enabled (REUSEADDR) by all the sockets.
# Assume desired local port range is 60_000-60_511
s = socket(AF_INET, SOCK_STREAM)
s.setsockopt(SOL_SOCKET, SO_REUSEADDR, 1)
s.bind(("192.0.2.1", 60_000))
s.connect(("1.1.1.1", 53))
# Fails only if 192.0.2.1:60000 -> 1.1.1.1:53 is busy
# Application must retry with another local port
In case of UDP, the network stack allows binding more than one socket
to the same 4-tuple, when local port sharing is enabled
(REUSEADDR). Hence detecting the conflict is much harder and involves
querying sock_diag and toggling the REUSEADDR flag [1].
b) For TCP, bind()-ing to a port within the ephemeral port range means
that no connecting sockets, that is those which leave it to the
network stack to find a free local port at connect() time, can use
the this port.
IOW, the bind hash bucket tb->fastreuse will be 0 or 1, and the port
will be skipped during the free port search at connect() time.
2. Isolate the app in a dedicated netns and use the use the per-netns
ip_local_port_range sysctl to adjust the ephemeral port range bounds.
The per-netns setting affects all sockets, so this approach can be used
only if:
- there is just one egress IP address, or
- the desired egress port range is the same for all egress IP addresses
used by the application.
For TCP, this approach avoids the downsides of (1). Free port search and
4-tuple conflict detection is done by the network stack:
system("sysctl -w net.ipv4.ip_local_port_range='60000 60511'")
s = socket(AF_INET, SOCK_STREAM)
s.setsockopt(SOL_IP, IP_BIND_ADDRESS_NO_PORT, 1)
s.bind(("192.0.2.1", 0))
s.connect(("1.1.1.1", 53))
# Fails if all 4-tuples 192.0.2.1:60000-60511 -> 1.1.1.1:53 are busy
For UDP this approach has limited applicability. Setting the
IP_BIND_ADDRESS_NO_PORT socket option does not result in local source
port being shared with other connected UDP sockets.
Hence relying on the network stack to find a free source port, limits the
number of outgoing UDP flows from a single IP address down to the number
of available ephemeral ports.
To put it another way, partitioning the ephemeral port range between hosts
using the existing Linux networking API is cumbersome.
To address this use case, add a new socket option at the SOL_IP level,
named IP_LOCAL_PORT_RANGE. The new option can be used to clamp down the
ephemeral port range for each socket individually.
The option can be used only to narrow down the per-netns local port
range. If the per-socket range lies outside of the per-netns range, the
latter takes precedence.
UAPI-wise, the low and high range bounds are passed to the kernel as a pair
of u16 values in host byte order packed into a u32. This avoids pointer
passing.
PORT_LO = 40_000
PORT_HI = 40_511
s = socket(AF_INET, SOCK_STREAM)
v = struct.pack("I", PORT_HI << 16 | PORT_LO)
s.setsockopt(SOL_IP, IP_LOCAL_PORT_RANGE, v)
s.bind(("127.0.0.1", 0))
s.getsockname()
# Local address between ("127.0.0.1", 40_000) and ("127.0.0.1", 40_511),
# if there is a free port. EADDRINUSE otherwise.
[1] https://github.com/cloudflare/cloudflare-blog/blob/232b432c1d57/2022-02-connectx/connectx.py#L116
v3 -> v4:
* Clarify that u16 values are in host byte order (Neal)
v2 -> v3:
* Make SCTP bind()/bind_add() respect IP_LOCAL_PORT_RANGE option (Eric)
v1 -> v2:
* Fix the corner case when the per-socket range doesn't overlap with the
per-netns range. Fallback correctly to the per-netns range. (Kuniyuki)
Reviewed-by: Marek Majkowski <marek@xxxxxxxxxxxxxx>
Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
---
include/net/inet_sock.h | 4 ++++
include/net/ip.h | 3 ++-
include/uapi/linux/in.h | 1 +
net/ipv4/inet_connection_sock.c | 25 +++++++++++++++++++++++--
net/ipv4/inet_hashtables.c | 2 +-
net/ipv4/ip_sockglue.c | 18 ++++++++++++++++++
net/ipv4/udp.c | 2 +-
net/sctp/socket.c | 2 +-
8 files changed, 51 insertions(+), 6 deletions(-)
diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index bf5654ce711e..51857117ac09 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h
@@ -249,6 +249,10 @@ struct inet_sock {
__be32 mc_addr;
struct ip_mc_socklist __rcu *mc_list;
struct inet_cork_full cork;
+ struct {
+ __u16 lo;
+ __u16 hi;
+ } local_port_range;
};
#define IPCORK_OPT 1 /* ip-options has been held in ipcork.opt */
diff --git a/include/net/ip.h b/include/net/ip.h
index 144bdfbb25af..c3fffaa92d6e 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -340,7 +340,8 @@ static inline u64 snmp_fold_field64(void __percpu *mib, int offt, size_t syncp_o
} \
}
-void inet_get_local_port_range(struct net *net, int *low, int *high);
+void inet_get_local_port_range(const struct net *net, int *low, int *high);
+void inet_sk_get_local_port_range(const struct sock *sk, int *low, int *high);
#ifdef CONFIG_SYSCTL
static inline bool inet_is_local_reserved_port(struct net *net, unsigned short port)
diff --git a/include/uapi/linux/in.h b/include/uapi/linux/in.h
index 07a4cb149305..4b7f2df66b99 100644
--- a/include/uapi/linux/in.h
+++ b/include/uapi/linux/in.h
@@ -162,6 +162,7 @@ struct in_addr {
#define MCAST_MSFILTER 48
#define IP_MULTICAST_ALL 49
#define IP_UNICAST_IF 50
+#define IP_LOCAL_PORT_RANGE 51
#define MCAST_EXCLUDE 0
#define MCAST_INCLUDE 1
diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index d1f837579398..1049a9b8d152 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -117,7 +117,7 @@ bool inet_rcv_saddr_any(const struct sock *sk)
return !sk->sk_rcv_saddr;
}
-void inet_get_local_port_range(struct net *net, int *low, int *high)
+void inet_get_local_port_range(const struct net *net, int *low, int *high)
{
unsigned int seq;
@@ -130,6 +130,27 @@ void inet_get_local_port_range(struct net *net, int *low, int *high)
}
EXPORT_SYMBOL(inet_get_local_port_range);
+void inet_sk_get_local_port_range(const struct sock *sk, int *low, int *high)
+{
+ const struct inet_sock *inet = inet_sk(sk);
+ const struct net *net = sock_net(sk);
+ int lo, hi, sk_lo, sk_hi;
+
+ inet_get_local_port_range(net, &lo, &hi);
+
+ sk_lo = inet->local_port_range.lo;
+ sk_hi = inet->local_port_range.hi;
+
+ if (unlikely(sk_lo && sk_lo <= hi))
+ lo = max(lo, sk_lo);
+ if (unlikely(sk_hi && sk_hi >= lo))
+ hi = min(hi, sk_hi);
+
+ *low = lo;
+ *high = hi;
+}
+EXPORT_SYMBOL(inet_sk_get_local_port_range);
+
static bool inet_use_bhash2_on_bind(const struct sock *sk)
{
#if IS_ENABLED(CONFIG_IPV6)
@@ -316,7 +337,7 @@ inet_csk_find_open_port(const struct sock *sk, struct inet_bind_bucket **tb_ret,
ports_exhausted:
attempt_half = (sk->sk_reuse == SK_CAN_REUSE) ? 1 : 0;
other_half_scan:
- inet_get_local_port_range(net, &low, &high);
+ inet_sk_get_local_port_range(sk, &low, &high);
high++; /* [32768, 60999] -> [32768, 61000[ */
if (high - low < 4)
attempt_half = 0;
diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 7a13dd7f546b..e41fdc38ce19 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -1016,7 +1016,7 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
l3mdev = inet_sk_bound_l3mdev(sk);
- inet_get_local_port_range(net, &low, &high);
+ inet_sk_get_local_port_range(sk, &low, &high);
high++; /* [32768, 60999] -> [32768, 61000[ */
remaining = high - low;
if (likely(remaining > 1))
diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 9f92ae35bb01..b511ff0adc0a 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -923,6 +923,7 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
case IP_CHECKSUM:
case IP_RECVFRAGSIZE:
case IP_RECVERR_RFC4884:
+ case IP_LOCAL_PORT_RANGE:
if (optlen >= sizeof(int)) {
if (copy_from_sockptr(&val, optval, sizeof(val)))
return -EFAULT;
@@ -1365,6 +1366,20 @@ int do_ip_setsockopt(struct sock *sk, int level, int optname,
WRITE_ONCE(inet->min_ttl, val);
break;
+ case IP_LOCAL_PORT_RANGE:
+ {
+ const __u16 lo = val;
+ const __u16 hi = val >> 16;
+
+ if (optlen != sizeof(__u32))
+ goto e_inval;
+ if (lo != 0 && hi != 0 && lo > hi)
+ goto e_inval;
+
+ inet->local_port_range.lo = lo;
+ inet->local_port_range.hi = hi;
+ break;
+ }
default:
err = -ENOPROTOOPT;
break;
@@ -1743,6 +1758,9 @@ int do_ip_getsockopt(struct sock *sk, int level, int optname,
case IP_MINTTL:
val = inet->min_ttl;
break;
+ case IP_LOCAL_PORT_RANGE:
+ val = inet->local_port_range.hi << 16 | inet->local_port_range.lo;
+ break;
default:
sockopt_release_sock(sk);
return -ENOPROTOOPT;
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 9592fe3e444a..c605d171eb2d 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -248,7 +248,7 @@ int udp_lib_get_port(struct sock *sk, unsigned short snum,
int low, high, remaining;
unsigned int rand;
- inet_get_local_port_range(net, &low, &high);
+ inet_sk_get_local_port_range(sk, &low, &high);
remaining = (high - low) + 1;
rand = get_random_u32();
diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index a98511b676cd..b91616f819de 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -8322,7 +8322,7 @@ static int sctp_get_port_local(struct sock *sk, union sctp_addr *addr)
int low, high, remaining, index;
unsigned int rover;
- inet_get_local_port_range(net, &low, &high);
+ inet_sk_get_local_port_range(sk, &low, &high);
remaining = (high - low) + 1;
rover = get_random_u32_below(remaining) + low;
--
2.39.0
